Reinforced concrete is a widely-accepted material of construction. Severe deterioration of many reinforced concrete structures has been observed with increasing frequency throughout the United States. This problem is widespread, affecting not only transportation infrastructure components, but also “snowbelt” structures, as well as coastal buildings and related structures. The safety and longevity of such reinforced concrete structures are of prime concerns, and methods of protection are therefore important.
The basic problem associated with the deterioration of conventional reinforced concrete due to corrosion of embedded reinforcement is generally not that the reinforcing material itself is reduced in mechanical strength, but rather that the concrete cracks. Concrete contains pores that are interconnected throughout its structure, and this extensive network leads to permeability of the concrete to both liquids and gases. This is of critical importance in the corrosion process, because both the initiators (generally, chloride ion) and the supporters (for example, oxygen) of corrosion of the reinforcing steel must diffuse through the overlying concrete to the steel. Chlorides from natural sources such as salt water or application of deicing salts can penetrate concrete and attack the reinforcing steel of a bridge structural element.
Cracking of concrete can lead to problems regarding structural soundness (on, for example, pilings), to discomfort (for example, chuckholes in bridges), or to cosmetic problems (as in the case of facades on buildings). As an example, many elements of bridge structures are exposed to harsh environmental conditions. Consequently, the service life of bridges is often reduced. The number of deficient bridges needing repair as a result is enormous. There are more than 600,000 bridges in U.S. and about 25% of them need retrofit or replacement.
Certain additives have been used in attempts to improve the performance of reinforced concrete. Latex-modified concrete essentially uses a polymer emulsion in the mix-water which apparently impedes the penetration of surface chlorides into the concrete (and possibly oxygen diffusion through the concrete as well). Incorporation of wax compounds has also been used to create “internally sealed” concrete. In this approach, a heating cycle is employed following curing of the cement to form a hydrophobic layer of wax to form on the pore walls, which prevents ingress of surface chemicals.
More recently, ultra-high performance concrete (UHPC) has become an important structural material. UHPC benefits from being a “minimum defect” material—a material with a minimum amount of defects such as micro-cracks and interconnected pores with a maximum packing density. Several types of UHPC have been developed in different countries and by different manufacturers. The four main types of UHPC are compact reinforced composites (CRC), multi-scale cement composite (MSCC), and reactive powder concrete (RPC). RPC is the most commonly available UHPC and one such product is currently marketed under the name Ductual® by Lafarge, Bouygues and Rhodia.
Portland cement is the primary binder used in conventional UHPC, but at a much higher proportion as compared to ordinary concrete or HPC. High proportions of tricalcium aluminate (C3A) and tricalcium silicate (C3S), and a lower Blaine fineness are desirable for conventional UHPC. The addition of silica fume serves to increase particle packing, flowability due to spherical nature, and pozzolanic reactivity with calcium hydroxide. Quartz sand with a maximum diameter of about 600 m is the largest constituent aside from the steel fibers. Both the ground quartz (about 10 m) and quartz sand contribute to the optimized packing by reducing the amount of water necessary to produce a fluid mix, and therefore, permeability, while polycarboxylate superplasticizer contributes to improving workability and durability. Finally, the addition of steel fibers aids in preventing the propagation of micro-cracks and macro-cracks and thereby limits crack width and permeability. Description of UHPC compositions can be found at, for example, U.S. Pat. Nos. 8,881,485 and 8,974,598 each of which is incorporated herein, in its entirety, by reference.
Current technology for repairing bridges typically involves removing the damaged concrete and reinforcing steel, and rebuilding and/or repairing the damaged area by replacing the steel and concrete. It is a time-consuming process, and may also result in significant impact to the traveling public during, for example, bridge closures caused by repair process.
Other available techniques employ fiber reinforced polymer, or FRP, sheets that wrap around the bridge elements needing repairs. One disadvantage of using FRP is that for flat surfaces, the repair procedures are cumbersome. Moreover, the material properties of FRP are very different from those of concrete, inevitably resulting in major structural challenges.
Development of a method to easily and quickly repair damaged structural elements is desirable for lowering the cost and improving the service life of bridges and other structures.